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Astronomy Links Telescope Basics
Here is a basic introduction to amateur telescopes. Types of Telescopes There are basically two types of telescopes, the refractor telescope and the reflector telescope. The refractor uses a front objective lens to collect light and focus it onto the eyepiece at the other end of the telescope. A second lens behind the main front lens is necessary to correct for false colors that would otherwise show up in the eyepiece.
The Newtonian reflector uses a mirror at the back end to collect and focus light onto a smaller secondary mirror which diverts the light into the eyepiece. The eyepiece is responsible for magnifying the image so it is visible to the human eye. There are several different types of reflector telescopes.
The Newtonian is described above. The Schmidt-Cassegrain reflector is like a combination of a reflector and refractor. A spherical mirror at the back end collects light and focuses it onto a small secondary at the front which then directs it back through a hole in the main primary mirror and into the eyepiece. The primary mirror in a SCT telescope produces severe aberrations so a corrector lens is placed at the front of the telescope to correct for these. The Maksutov-Cassegrain is similar to the Schmidt Cassegrain but has a different lens design to again correct for aberrations.
Telescope Mounts There are two types of mounts for telescopes: 1. The
Alt/Azimuth Mount The alt/azimuth mount moves the telescope only in altitude (up and down) and azimuth (right and left). It is not possible to accurately track star positions with this type of mount. Field rotation will occur when tracking for long periods. Many refractors and Schmidt-Cassegrain telescopes are fork mounted, which is a type of alt/azimuth mount. Large Newtonian reflectors are usually Dobsonian mounted, another type of alt/azimuth mount.
The equatorial mount moves the telescope in right ascension and declination. This mount can accurately track the position of a star if a clock drive motor is fitted to the right ascension axis. Almost any type of telescope can be placed on an equatorial mount.
Eyepieces Plossl Eyepieces - The best type of eyepiece for amateur astronomy. A 4 element design that gives sharp images with a wide field of view and good eye relief. Eyepieces are identified by focal length. Popular focal lengths are 10mm and 25mm. The shorter the focal length, the higher the magnification. Magnification
= The highest effective magnification in a telescope is 50 x aperture in inches. Barlow Lens This inserts between tithe focuser and eyepiece and doubles the magnification of the eyepieces. Magnification In astronomy, the resolution of the telescope, or light gathering ability, is more important than the magnification. Although a 60mm refractor might be capable of 400x magnification, the effective magnification is only 50 x 2.4inches = 120x as the lens does not have good light gathering ability. The highest effective magnification in a telescope is 50 x aperture in inches. i.e. 8inch (200mm) Newtonian = 50 x 8 = 400x To calculate the actual magnification of a telescope, use the following formula: Magnification
= i.e. 200mm f/6 with 10mm eyepiece mag = (200 x 6)/10 = 120x Focal Length The focal length of a telescope is the distance the primary lens focuses light to a single point. Focal length is often expressed as an f/ratio: f/ratio = focal length of lens/aperture of lens i.e. 200mm lens with a focal length of 1200mm f/ratio = 1200/200 = f/6 Collimation Newtonian telescopes can easily be knocked out of alignment. When this happens, the mirrors need to be recentered so the optic axis coincides with the optic axis of the eyepiece. To do this, a collimating eyepiece is used.  email Text and Images Copyright ©2004-2012 by Rob Kanen |
